The present invention relates to a hydraulic shock absorber to be mounted on a suspension apparatus for a vehicle, such as an automobile.
Generally, in a suspension apparatus for a vehicle such as an automobile, a hydraulic shock absorber is provided between a sprung mass and an unsprung mass, to thereby damp vibrations of the suspension apparatus to provide good ride quality and steering stability.
In a vehicle having a relatively large loading capacity, such as a van and a station wagon, a change in vehicle height occurs due to a change in the load on the vehicle, for example, a change in the number of passengers or the number of articles carried. Such a change in vehicle height leads to a lowering of ride quality and steering stability. Therefore, it has been desired to develop a suspension apparatus having an ability to automatically maintain a predetermined vehicle height regardless of the load carried by the vehicle.
For obtaining a suspension apparatus which automatically maintains a predetermined vehicle height, for example, it has been proposed to use a so-called self-pumping type hydraulic shock absorber. Such a self-pumping type hydraulic shock absorber is disclosed in Unexamined Japanese Patent Application Public Disclosure Kokai No. 60-261713. This hydraulic shock absorber comprises: an oil tank and a reservoir each having a gas and a hydraulic fluid sealably contained therein under appropriate pressure; pumping means for supplying the hydraulic fluid in the oil tank into a cylinder in accordance with extension strokes and compression strokes of a piston rod; and return means for returning the hydraulic fluid in the pumping means and the cylinder to the oil tank depending on the position of the piston rod during the strokes. In this hydraulic shock absorber, the pumping means and the return means are operated by utilizing vibrations of the suspension apparatus during the running of the vehicle, thereby controlling the pressure in the cylinder appropriately and adjusting the degree of extension of the piston rod to a predetermined level, thus automatically maintaining a predetermined vehicle height.
In the self-pumping type hydraulic shock absorber of Kokai No. 60-261713, a double-cylinder structure is employed by providing an outer cylinder to cover an outer circumferential surface of the above-mentioned cylinder, and the oil tank and the reservoir are provided between these two cylinders. In this case, the volume of each of the oil tank and the reservoir is inevitably small and, therefore, when the load carried by the vehicle is large, the rate of increase in repulsive force of the gas during the strokes of the piston rod is large, leading to a lowering of ride quality.
For suppressing a change in repulsive force of the gas during the strokes of the piston rod to provide good ride quality, it has been desired to employ an oil tank and a reservoir each having a sufficiently large volume. An oil tank and a reservoir each having a large volume can be employed by providing the oil tank separately from the main body of the hydraulic shock absorber. However, for example, in a suspension apparatus for an automobile, the space for mounting of the hydraulic shock absorber is very limited, making it difficult to secure the space that is necessary for accomodating the separate oil tank.
The present assignee proposed, in Japanese Patent Application No. 7-331050 [corresponding to Unexamined Japanese Patent Application Public Disclosure (Kokai) No. 9-144801], a self-pumping type hydraulic shock absorber in which an annular oil tank is provided around an outer cylinder below a spring sheet which is attached to the outer cylinder to receive a suspension spring.
However, in the hydraulic shock absorber in which the oil tank is provided below the spring sheet, the length of the suspension spring is limited for securing the space for mounting of the oil tank. Further, the outer diameter of the hydraulic shock absorber around its portion to be connected to the suspension apparatus is large, so that the shape of the suspension apparatus at its portion to be connected to the hydraulic shock absorber is limited.